Metal strips with metallic coatings have attained great importance in many areas of technology. As composite materials they offer the possibility of combining the special properties of the basic material, such as for example strength or magnetic properties, with special properties of the coating material such as for example corrosion resistance or decorative appearance. Electrolytic metal deposition represents a possibility for manufacturing metallic coatings with small thickness tolerances, high surface quality and without the influence of the technological properties of the base material. Examples of such electrolytically plated metal strips are thin steel sheet with an electrolytically deposited lead-tin alloy such as used for fuel containers or a thin steel sheet electrolytically zinc plated or zinc alloy plated for corrosion protection for vehicle bodies as well as electrolytically zinc plated thin steel sheet for manufacturing containers and sheet metal packages.
The largest economic importance resides in the electrolytic zinc plating or zinc alloy plating of thin sheet material for rust protection in the automobile industry. Whereas steel sheet plated with pure zinc is to a large extent cathodically protected by the zinc coating, zinc alloy plated steel sheet exhibits a larger barrier protection effect with respect to attacking corrosive media. The provision of one or the other of the two named zinc coating types is dependent on the weight that the individual automobile manufacturer places on the kinds of corrosion protection.
In earlier times, in the case of electrolytic zinc plating of strips there appeared a trend to installations with vertical movement of the strip in the electrolysis area (vertical cell apparatus). However, worldwide there are also many installations in operation for electrolytic metal deposition on metal strips in which the electrolysis area is arranged horizontally (horizontal cell apparatus), or the electrolytic processing of the metal strip takes place on one side, which strip is deflected by an immersion roll which roll if it is made with a metallic outer surface can also serve as the cathode contact means for the metal strip to be plated (radial cell apparatus).
Strip processing apparatus using the radial cell principle require a double number of cells if the steel band is to be plated with zinc on both sides, which along with technical problems relating to the apparatus also brings with it accompanying large burdens from the investment side. Horizontal cell apparatus have disadvantages in the electrolysis area in that particles from the electrolyte can become deposited on the upper side of the strip, which particles become embedded in the upper coat, and in that on the underside of the strip oxygen collects in the case of less than 100% cathodic efficiency and hydrogen collects in the case of use of insoluble anodes, the gas collecting in the form of bubbles or blisters, which bubbles or blisters can come to disturb the deposition process. In the case of vertical cell apparatus particles engaging the outer surface of the strip are washed from the side of the ascending strip portion by the surge of the electrolyte flowing downwardly along the strip; the metal deposition can take place in one electrolysis cell either on one side, on two sides, or with different coating thicknesses, without the need for large conversion measures. Preferrably soluble anodes are employed, but insoluble anodes can also be used. Edge masks can be used. The process as well as the strip can be visually controlled in each of the employed electrolysis cells and the length of the apparatus is shorter than it is in the case of using either of the other two types of cell construction.
In the case of vertical cell apparatus, in the first generation a very simple construction was employed in which the anodes were arranged to hang freely on both sides of the descending and ascending runs of the strip, the electrolyte was delivered at a low flow rate into the lower region of the electrolysis cell and was discharged in the upper region over an overflow so that in the electrolysis cell electrolyte flow of a speed relevant to the electrolysis process existed. In the case of vertical cell apparatus of the second generation, in the apparatus, which includes immersion rolls, the lower region of the electrolysis cell is separated from the upper region which contains the anodes and therewith the electrolysis path containing cell areas, and the upper and lower regions are connected only through two slot shaped openings which in their short dimension correspond to the spacing between the anodes and in their long dimension correspond to the maximum anode width, through the middle of which slots the steel band to be finished is guided.
Associated with these openings are flow canals in the upper cell portion both for the descending as well as for the ascending strip portion, which canals are formed in the anodes or in plates arranged on the rear sides of the anodes and which in reference to the direction of movement of the strip are formed in the lateral container forming walls of the electrolysis cell. The electrolyte is delivered to the lower cell portion and flows through, in a laminar flow condition, flow compartments arranged in the upper cell portion, and is finally returned over an overflow into a collection container, from which a pump pumps the electrolyte back to the electrolysis cell. In this kind of apparatus the guiding of the flow is such as to produce at the descending strip portion a counterflow opposite to the direction of strip moment and at the ascending strip portion a flow in the same direction as the strip movement, whereby in the case of zinc plating a current density of 60 A/dm.sup.2 can be employed without producing a dendritic deposition of zinc.
If however, in the case of electrolytic zinc plating with high current densities, such as for example 100 A/dm.sup.2, zinc coatings with optimal zinc crystal structure are to be manufactured or zinc alloy coatings with similar formation of the alloy are to be made, vertical cells with large flow velocity of the electrolyte and similar flow rates at both rows of the strip are used. Since in comparison with one another the electrolysis conditions are more favorable in the electrolyte stream which moves counter to the strip direction then they are in the stream which runs with the strip direction, the problem arises of turning around in comparison to previously used cells the flow direction in the flow compartment with the ascending band portion and of raising the flow velocity of the electrolyte in both flow channels so that a turbulent flow condition exists.
Such a counterflow cell is known from UK patent application GB No. 2,147,009A. The electrolysis cell described in this application is further characterized by the exclusive use of insoluble anodes, by the circulation of the entire electrolyte mass flowing in the flow channels by means of external pumps and by the guiding of the pumped electrolyte mass through jet tubes arranged perpendicular to the strip movement direction, which jet tubes are located on both sides of the strip at the inflow side of the flow channels.
Along with the grounds mentioned in the UK application for the choice of insoluble anodes there are however also many disadvantages for this type of anode which can be avoided through the choice of soluble anodes.
In the case of zinc plating with the use of insoluble anodes a separate dissolving circuit for the follow up of the zinc ions in the electrolyte is necessary, as a result of larger anode polarization there exists a higher voltage loss between anode and cathode, and a large amount of oxygen is developed at the insoluble anodes which fills a portion of the space between the anode and cathodes and so leads to a further increase in the electrolysis voltage and unfavorably influences the coating quality. Further, the most expensive insoluble anodes are of only limited strength and are easily destroyed if as a result of engagement with the steel strip electrically connected to the cathode a short circuit arises, which in the case of the use soluble anodes produces no cost with regard to anodes. If as in the UK application the insoluble anodes are manufactured from lead alloys a small amount of lead is deposited with the zinc and leads after heat treatment of the plated steel sheet to loosening of the coating if in the electrolysis cycle expensive handling of the electrolyte is not provided in order to remove lead gone into solution at the anode site. If lead oxide particles exist at the insoluble anodes due to blowing off of the oxide cover this can negatively influence the coating quality. Further the large amount of oxygen existing because of the strived for high current density has as a result that aerosols are delivered from the electrolysis cell and make necessary an especially extensive exhaust system. The use of soluble anodes in strip form allows the width of the anodes to be easily suited to the width of the strip which leads to a favorable coating distribution on the steel band. Such soluble anodes are used in high production zinc plating apparatus with the help of mechanized handling devices in the electrolysis cells and they are withdrawn in worn out condition. Accordingly, down time for the apparatus is relatively small as is the need for personnel.
In alloy zinc plating with soluble anodes basket anodes have come into use, in which case the anode space is so separated from the cathode by a diaphragm that particles of the anode material which originate in the basket cannot succeed in reaching the metal strip to be processed. Since in modern band processing plants a large number of electrolysis cells are used the number of anode baskets filled with one of the alloying elements of the coating can be so chosen that the ionic relationship of the electrolyte can be held constant by the addition of only a small amount of salt, in which case it is advantageous to use for the correction a salt of the alloying element which together with the zinc is to deposited as the alloyed coating.
Just as in the case of pure zinc plating, in the case of alloy zinc plating with soluble anodes all customary types of electrolytes, such as for example choloride electrolyte, can be used which in general exhibit a higher electrolytic conducting ability than sulfate electrolyte, which in the case of using insoluble anodes has come to be used exclusively.
In the case of electrolytic deposition of tin and of lead-tin alloys the use of insoluble oxygen anodes leads to a rapid formation of tetravalent tin and therewith a large loss of tin through tin matte formation. Moreover, the customarily used fluoroborate electrolyte can not be used together with insoluble anodes.
The circulation of the entire flow mass through external pumps demands large tube cross sections for the electrolyte delivery into the electrolysis cells, which in general can be suited only in complicated ways to the cell geometry determined by the diameters of the reversing rolls and by the anode length.
The arrangement of the delivery tubes provided with jets above the flow compartment with the ascending band portion leads to a large stretch between the current roll and the electrolysis region, whereby an unnecessarily large voltage loss arises in the material strip to be finished. The consumed energy for the circulatory pumping of the entire electrolyte flow mass through external pumps and through, for example, heat exchangers, filters and pump reservoirs is extraordinarily high, since moreover large distances and high differences due to constructional limitations have to be overcome.
It is further very difficult to conduct a fluid stream created by a pump and delivered through a tube at right angles around a symmetrical surface while at the same time creating a very wide stream. The difficulty at arriving at a solution finds expression in the UK patent application which proposes the most different constructions for the formation of the jet tubes.